1. Field of the Invention
The present invention relates to cross-point switches and more particularly the application of phase change switches to cross-point switches.
2. Description of Background
Three major types of reconfigurable logic and circuit fabrics are transistor based cross-point switches, fuse based cross-point switches, and anti-fuse based cross-point switches. The transistor based cross-point switches can be reprogrammed multiple times, whereas the fuse based and the anti-fuse based cross-point switches are typically only one time reprogrammable.
Conventional transistor based cross-point switches require wiring each individual terminal node in a cross-point switch to another individual terminal node in the cross-point switch. Additionally, each pair of terminal nodes requires an individual switch assigned to regulate the connection between the pair of terminal nodes. Current transistor based cross-point switch requires,
      ∑          k      =      1              n      -      1        ⁢  kconnections/transistor switches, where n is the number of terminal nodes in the transistor cross-point switch. Thus, the number of “moving parts” and overall resistance of the cross-point switch is greatly increased with each additional terminal node.
Fuse based reconfiguration technology presently relies on several methods to make (“fuse”) or break (“antifuse”) electrical connections in fabricated structures. For example, laser-fusible links represent an early approach, which are now replaced by electrical techniques entirely internal to the chip. In addition, electro-migration fuses (such as in IBM's eFUSE technology for rerouting chip logic), are currently in use. An electro-migration fuse takes up a relatively large area and requires a high current to blow the fuse. Also, an electro-migration fuse is “one-shot” (as stated above) in that once the fuse is blown, it cannot be returned to a conducting state. Furthermore, the variation of eFUSE characteristics is relatively broad, thus requiring that the state of each fuse to be sensed by a discriminating circuit with the digital result stored in a latch. The blowing of an electro-migration fuse is also relatively slow. Additionally, similar to the current transistor-based cross-point switches often require a fuse region for each pair of terminal nodes.
Anti-fuse approach (e.g., used for some DRAM repair operations) typically involves a very thin dielectric material such as silicon dioxide, or a sandwich combination of silicon oxide-nitride-oxide (ONO), between two conductors. Anti-fuse programming is performed by applying a relatively high voltage through the conducting terminal. This causes dielectric breakdown in the dielectric so that the resistance of the anti-fuse permanently changes from high to low. This is also a one-shot technique requiring high voltage, as mentioned above. Again, similar to the current transistor-based cross-point switches often require an anti-fuse region for each pair of terminal nodes.
Unfortunately, the existing controllable link technologies described above may not have optimal properties for future microchip generations, due to factors such as: excessive area taken up by the fuse, “sunsetting” of the non-standard high voltages/currents which may be required by fuse programming, the desirability of “multishot” reprogrammable fuses, and insufficient speed relative to application specific integrated circuit (ASIC) designs.
Accordingly, it would therefore be desirable to provide a reprogrammable cross-point switch suitable with performance similar to ASIC designs.